SAMPEX observations of energetic electron precipitation in the dayside low-latitude boundary layer

We address the problem of the magnetic topology of the dayside low-latitude boundary layer by using conjugate observations from the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) and the Defense Meteorological Satellite Program Flight 10 (DMSP F10) satellites while they are in coplanar polar orbits in the 0900-2100 magnetic local time (MLT) meridian. In the region identified in the DMSP observations as the low-latitude boundary layer (LLBL), we observe energetic electron precipitation at SAMPEX in detectors measuring electrons in the energy ranges E > 150 keV and E > 1.05 MeV. This precipitation is in the region that would otherwise be identified as the polar cap. We find that the equatorward edge of the energetic electron precipitation coincides precisely with the equatorward edge of the LLBL as these boundaries move over a wide range of invariant latitude. We investigate the hypothesis that the energetic electron precipitation is due to pitch angle scattering in the magnetopause current layer of magnetospheric electrons on open drift paths and find that this hypothesis is reasonable. Additional evidence supporting this hypothesis is the dependence of the precipitating flux on the solar wind clock angle. A consequence of this hypothesis is that the low-latitude boundary layer in the 0900-1000 MLT sector lies completely on open magnetic field lines except, perhaps, when the interplanetary magnetic field is predominantly northward.